A power supply of an electronic device such as a television, a smart phone, a PC and the like should stably supply a constant direct current voltage. As the power supply, a switching power-supply device is used (for example, refer to JP-A-9-308243 and JP-A-2002-209383).
A switching power-supply device uses, as a switching element, a semiconductor device such as a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), an IGBT (Insulated Gate Bipolar Transistor), a thyristor and the like to convert an input direct current voltage into an alternating current voltage by an on-off operation of the switching element. The converted alternating current voltage passes through a transformer, a rectification circuit and a smoothing circuit in corresponding order, is converted into a stable direct current voltage and is then output.
The switching power-supply device disclosed in JP-A-9-308243 is a current resonance-type converter and performs control of returning a frequency of a switching signal into an appropriate operating range when the frequency of the switching signal becomes lower than a resonance frequency and an off-resonance state is made.
The switching power-supply device disclosed in JP-A-2002-209383 has a full bridge-type converter and controls an output voltage by controlling on-off operations of four switching units configuring the full bridge.